Adaptive quantization is used by frequency-domain audio encoders, such as the advance audio coding (AAC) and MP3 encoder, to reduce the number of bits required to store encoded audio data, while maintaining a desired audio quality.
Adaptive quantization transforms time-domain digital audio signals into frequency-domain signals and groups the respective frequency-domain spectrum data into frequency bands, or scalefactor bands. In this manner, the techniques used to eliminate redundant data, i.e., inaudible data, and the techniques used to efficiently quantize and encode the remaining data, can be tailored based on the frequency and/or other characteristics associated with the respective scalefactor bands, such as the perception of the frequencies in the respective scalefactor bands by the human ear.
For example, in advance audio coding, the interval, or scalefactor, used to quantize each respective scalefactor band can be individually determined for each scalefactor band. Selection of a scalefactor for each scalefactor band allows the advance audio coding process to use scalefactors to quantize the signal in certain spectral regions (the scalefactor bands) to leverage the compression ratio and the signal-to-noise ratio in those bands. Thus scalefactors implicitly modify the bit-allocation over frequency since higher spectral values usually need more bits to be encoded. The use of larger scalefactors reduces the number of bits required to encode a scalefactor band, however, the use of larger scalefactors introduces an increase amount of distortion to the encoded signal. The use of smaller scalefactors decreases the amount of distortion introduced to the final encoded signal, however, the use of smaller scalefactors also increases the number of bits required to encode a scalefactor band.
In order to achieve improved sound quality as well as improved compression, selection of an appropriate scalefactor for each scalefactor band is an important process. Unfortunately, current approaches for selecting a scalefactor for a scalefactor band are computationally complex and processor cycle intensive.
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